1. Field of the Invention
This invention relates to pressure responsive devices for actuating movable elements such as electric switches, meter pointers or the like.
2. Description of the Prior Art
A pressure responsive device as heretofore known employs a diaphragm between a source of gas or liquid under pressure and a mechanism to be actuated at a predetermined pressure of such fluid for controlling the condition of a device such as an electric switch. See, for example, my U.S. Pat. No. 2,824,919, "Pressure Responsive Switch, " of Feb. 25, 1958. In such prior art switches, a snap action negative rate disc spring causes the switch device to be closed at a predetermined fluid pressure, and to be opened at a second predetermined fluid pressure. An adjustable compression spring is provided with which to cause the device to work within a desired pressure range. The pressure differential for opposite disc spring movements is the deadband, i.e., the difference between the pressure that causes the disc spring to snap to depress the extended switch plunger and the pressure at which the disc spring snaps back to permit the switch plunger to return to extended position. However, such pressure responsive devices are characterized in that their deadband varies considerably at different pressure levels. In this regard, a phenomenon of a diaphragm is that it alone causes a substantial variation in deadband at different pressure ranges or levels, with the deadband increasing substantially with increase in pressure, e.g., from 10-psi in the vicinity of 100-110 psi applied pressure to 75-psi in the vicinity of 925-1000 psi applied pressure. The combined effects of a negative rate snap action disc spring and a positive rate compression spring in a device having such a diaphragm are not able to compensate for the effect of the apparent variable spring rate of the diaphragm. Rather, such variable rate of the diaphragm has been recognized and viewed as an inherent limitation of pressure responsive devices employing them, as to which no solution has existed to easily and simply remove the same.